The present invention relates to an electroluminescent device, such as a self-luminescent plane display, and, more particularly, to an electroluminescent device suitable for an organic electroluminescent display employing an organic thin film as an electroluminescent layer.
The importance of man-machine interfaces including multimedia products has increased in recent years. It is necessary to take out a sufficient quantity of information correctly, simply and instantaneously from a machine to enable an operator to operate the machine efficiently and agreeably. Efforts have been made to develop various display devices. The progressive miniaturization of machines has amplified demand for the reduction in size and thickness of display devices. For example, the miniaturization of lap-top information processors integrally provided with a display device, such as notebook personal computers and notebook word processors has made an astonishing progress in recent years, and marvelous technological innovation has been achieved in liquid crystal displays suitable for such lap-top information processors to deal with the miniaturization of lap-top information processors.
In these days, the liquid crystal display is used as an interface for various daily products including lap-top information processors, small television receivers, watches and pocket calculators, not to mention lap-top information processors. Liquid crystal displays are characterized by their capability of being driven by power of a relatively low voltage and of operating at a relatively low power consumption. Studies of display devices have been made to develop display devices having large capacities as well as small capacities for use in man-machine interfaces. However, a liquid crystal display needs more power for driving its backlight unit than for driving its liquid crystal unit because the liquid crystal display is not self-luminescent. Therefore, the liquid crystal display is able to operate only a short period of time where powered by an internal battery or the like, which is a restriction on the use of the liquid crystal display. The liquid crystal display is not suitable for use as a large display because the liquid crystal display has a narrow viewing angle. It is a significant problem in the liquid crystal display that the contrast of a picture displayed on the liquid crystal display varies with the angle of viewing direction even within the viewing angle because the liquid crystal display forms pictures by changing the orientation of liquid crystal molecules. Although an active matrix driving system is able to operate at a response speed sufficiently high to deal with animated pictures, defects in pixels makes the formation of a large screen impossible because the active matrix driving system employs a thin-film transistor (TFT) driving circuit. The employment of a TFT driving circuit is not desirable from the viewpoint of cost reduction.
A simple matrix driving system is inexpensive and a liquid crystal display of a simple matrix driving system having a large screen size can be relatively easily fabricated. However, the response speed of the simple matrix driving system is not high enough to deal with animated pictures.
Studies have been made to develop self-luminescent display devices, such as plasma display devices, inorganic electroluminescent displays and organic electroluminescent displays. The plasma display device uses plasma light produced by discharge in a low-pressure gas and is suitable for forming large displays of a large capacity. However, the plasma display device has problems in thickness reduction and cost reduction. The plasma display device needs a high ac bias voltage and hence the plasma display device is not suitable for use in portable machines. The inorganic electroluminescent device is commercially used in displays of green luminescence. However, the inorganic electroluminescent device, similarly to the plasma display device, needs a high ac bias voltage and needs driving power of several hundreds of volts, which is disadvantageous to the practical application of the inorganic electroluminescent device.
The development of relevant technology has enabled the inorganic electroluminescent device to display color pictures in three primaries, i.e., red (R), green (G) and blue (B). However, since the inorganic electroluminescent device uses an inorganic material, it is difficult to control the wavelength of light to be emitted by the inorganic electroluminescent device and to display full-color pictures. Studies of electroluminescence of an organic compound have been made for a long time since the discovery of luminescence of an anthracene crystal resulting from injection of carriers into the anthracene crystal. However, since the luminescence of an anthracene crystal is of a low luminance and monochromatic, only basic studies of carrier injection into an organic material have been made.
Tang et al. of Eastman Kodak published an organic thin-film electroluminescent device of a laminated structure having an amorphous luminescent layer capable of emitting light in a high luminance and of being driven by power of a low voltage in 1987. Since then, active studies have been made to achieve light emission in three primaries, i.e., red, green and blue, stable light emission and luminance enhancement, and efforts have been made to develop suitable laminated structures and fabricating processes.
New organic materials have been invented through molecular design and active studies have been started to enable the application of thin organic electroluminescent display devices having excellent characteristics including self-luminescence to color displays.
The organic electroluminescent device (hereinafter, referred to also as "organic EL device") has a film of .mu.m or below in thickness which become luminescent by converting electric energy into optical energy when a current is supplied thereto, which is an ideal property for a self-luminescent display device.
FIG. 33 shows a conventional organic EL device 10 by way of example. The organic EL device 10 is fabricated by sequentially forming a transparent electrode 5 of ITO (indium tin oxide), i.e., an anode, a hole transport layer 4, a light emitting layer 3, an electron transport layer 2 and a cathode 1, such as an aluminum electrode, in that order on a transparent substrate 6, such as a glass plate by, for example, vacuum evaporation processes. When a dc voltage 7 is applied selectively across the transparent electrode 5, i.e., an anode, and the cathode 1, holes, i.e., charge carriers, injected by the transparent electrode 5 move through the hole transport layer 4, electrons injected by the cathode 1 move through the electron transport layer 2, and holes and electrons recombine. When holes and electron recombine, light 8 of a predetermined wavelength can be seen through the transparent substrate 6.
The light emitting layer 3 may be made of a luminescent substance, such as anthracene, naphthalene, phenanthrene, pyrene, chrysene, perylene, butadiene, coumarin, acridine, stilbene or the like. The electron transport layer 2 may contain such a luminescent substance.
FIG. 34 shows another conventional organic EL device 20. the organic EL device 20 is not provided with any layer corresponding to the light emitting layer 3 of the foregoing organic EL device 10, has an electron transport layer 2 containing a foregoing luminescent substance, and emits light 18 of a predetermined wavelength from an interface between the electron transport layer 2 and a hole transport layer 4.
FIG. 35 shows a concrete example of the foregoing organic EL device. A laminated structure of the organic layers, i.e., the hole transport layer 4, and the light emitting layer 3 or the electron transport layer 2, are sandwiched between the cathode 1 and the anode 5. The cathode 1 has conductive strips extending in a direction, and the anode 5 has conductive strips extending in a direction perpendicular to the conductive strips of the cathode 1. Points of overlap of the conductive strips of the cathode 1 and the anode 5 constitute pixels. A signal voltage is applied across the pixels by a luminance signal circuit 34 and a controller 35 with a built-in shift resister controls the signal voltage in a time series to make the pixels emit light. The organic EL device can be used as an image reproducing device and, naturally, as a display. If the organic EL device is provided with sets each of the intersecting conductive strips for R, G and B, respectively, the organic EL device can be used as a full-color or multicolor display. In a display employing such an organic EL device having a plurality of pixels, the layers 2, 3 and 4 of organic thin films, in general, are sandwiched between the transparent electrode 5 and the cathode 1, i.e., a metal electrode, and light is emitted through the transparent electrode 5.
However, the foregoing organic EL device has problems to be solved. For example, it is essential when applying an organic EL device to a color display that the organic EL device is capable of stably emitting light rays of three primary colors, i.e., red, green and blue. However, there has been no report about materials capable of stably emitting red or blue light rays of satisfactory chromaticity in a satisfactory luminance, only those that emit green light rays. At present, it is particularly difficult to emit blue light stably because blue light emission involves heat generation by a heat relaxation process accompanying light emission and the existence of singlet oxygen. A coloring matter having a high crystallinity produces a polymer when it solidifies, so that the wavelength of the light emitted by the coloring matter increases and the coloring matter is unable to sustain light emission.
Many studies have been made for the development of new blue light emitting materials. Achievement of stable light emission by existing materials is important as well as the research and development of new substances. The use of substances of established properties as materials contributes greatly to the curtailment of research and development activities and provides guidelines on the development of materials. A coumarin laser coloring matter having a high fluorescence yield can be used as a doping material for improving the color purity of green light emission. It is reported that a coumarin laser coloring matter could be used as a blue light emitting material. Although a coumarin short-wavelength fluorescent coloring matter as a simple substance, in general, has a high crystallinity and an amorphous coumarin short-wavelength fluorescent coloring matter is not a suitable, stable blue light emitting material, a stable amorphous thin film can be formed by coevaporation. For example, fluorescent light emitted by coumarin 450 has a maximum wavelength of about 446 nm and has chromaticity corresponding to blue. However, since coumarin does not have properties to transport electrons or holes, the light emitting characteristics of coumarin is apparently inferior to those of materials having properties to transport electrons or holes. When a material, typically, zinc complex, is used, stable blue light emission can be achieved by forming an electron transporting blue light emitting layer in a single heterostructure. However, the intensity of light having a spectrum around 700 nm in which visual sensitivity is high increases when the voltage of power is increased to make the blue light emitting layer emit light in a sufficient luminance. Consequently, the chromaticity of the blue light changes adversely and the blue light emitting layer tends to emit light in a white light emitting mode. Generally, since organic EL devices have a relatively short life, active studies have been made in various fields for the extension of the life of organic EL devices. Preferably, the organic EL device has a half value time, i.e., a time period in which its initial luminance (about 200 cd) decreases by half, of 10,000 hr or above to enable the organic EL device to be used as a display. However, such a long half value time has not yet been achieved, which is a significant problem to be solved to enable the practical application of organic EL devices.